1. Field of the Invention
The present invention generally relates to bearings having limited radial space for the hydrodynamic bearing housing and, more particularly, to a novel cage positioned tilting pad bearing having particular application in turbochargers and other high rotary speed devices.
2. Background Description
High speed turbochargers are intended to increase the power of internal combustion engines. The first turbocharger was invented in the early twentieth century by the Swiss engineer Alfred Buchi who introduced a prototype to increase the power of a diesel engine. Turbocharging was not widely accepted at that time, but in the last few decades, turbocharging has become standard for most diesel engines and is used in many gasoline engines as well. Since the earliest turbocharger prototypes, researchers have attempted to improve turbocharger reliability and increase turbocharger life (Born, H. R., “Analytical and experimental investigation of the stability of the rotor-bearing system of new small turbocharger,” in Proceedings of the Gas Turbine Conference and Exhibition, Anaheim, Calif., May 31-Jun. 4, 1987). Since vibration-induced stresses and bearing performance are major failure factors, rotordynamic analysis should have been an important part of the turbocharger design process. A thorough rotordynamic investigation was, however, very difficult and relatively few studies were published in the early years.
Advances in rotor dynamic analysis computer programs have now made the analysis of a turbocharger rotor-bearing system a reality (Gunter, E. J. and Chen, W. J., 2001, DyRoBeS—Dynamics of Rotor Bearing Systems User's Manual, RODYN Vibration Analysis, Inc., Charlottesville, Va.). Manufacturers have begun using these tools to better understand the dynamics of high speed turbochargers. Design improvements, however, cannot depend on computational analysis alone (Holmes, R., Brennan, M. J., and Gottrand, B., “Vibration of an automotive turbocharger—a case study,” in Proceedings of the 8th International Conference on Vibrations in Rotating Machinery, Swansea, UK, Sep. 7-9, 2004, pp. 445-450) and on-engine test data are still required for these still very difficult analytical predictions.
A previous investigation used a commercial finite element analysis (FEA) computer program to model the dynamics of the turbocharger (Alsaeed, A. A., 2005, “Dynamic Stability Evaluation of an Automotive Turbocharger Rotor-Bearing System,” M. S.
Thesis, Virginia Tech Libraries, Blacksburg, Va., and Kirk, R. G., Alsaeed, A. A. and Gunter, E. J., 2007, “Stability Analysis of a High-Speed Automotive Turbocharger,” Tribology Transactions, 50(3), pp 427-434). That investigation demonstrated how linear analysis can be beneficial for understanding the basic experimental dynamic performance of the turbocharger rotor bearing system. This current experimental research extends previous experimental work with on-engine testing (Andres, L. and Kerth, J., “Thermal effects on the performance of floating ring bearings for turbochargers,” Proceedings of the Institute of Mechanical Engineers Journal of Engineering Tribology 218(J), 2004, pp. 437-450, Kirk, R. G., Alsaeed, A., Liptrap, J., Lindsey, C., Sutherland, D., Dillon, B. et al., “Experimental test results for vibration of a high speed diesel engine turbocharger,” Tribology Transactions 51(4), 2008, pp. 422-427., and Kirk, R. G., A. Kornhauser, J. Sterling, and A. Alsaeed, 2010, “Turbocharger On-Engine Experimental Vibration Testing,” ASME Journal of Vibration and Control, 16(3): 343-355). The past testing of custom design fixed geometry design bearings, demonstrated the non-linear jump for no-load conditions at certain speeds (Kirk, R. Gordon, et al., “Influence of Turbocharger Bearing Design on Observed Linear and Nonlinear Vibration,” ASME/STLE IJTC2010-41021, San Francisco, Oct. 17-20, 2010).
A turbocharger consists basically of a compressor and a turbine coupled on a common shaft. The turbocharger increases the power output of an engine by compressing excess air into the engine cylinder, which increases the amount of oxygen available for combustion. Since the output of reciprocating internal combustion engines is limited by the oxygen intake, this increases engine power (Ward, D. et al., U.S. Pat. No. 6,709,160). Since the turbine is driven using energy from the exhaust, turbocharging has little effect on engine efficiency. By contrast, a supercharger using power from the engine shaft to drive a compressor also increases power, but with an efficiency penalty.
An important factor in the design of an automotive turbocharger is the initial cost. The same power increase provided by the turbocharger can be provided by simply building a larger engine. Since engine weight is not a major part of overall weight for a diesel truck, the turbocharger is only competitive if it is less expensive than increasing engine size. For passenger cars the turbocharged diesel must compete with lighter and less expensive gasoline engines. To keep costs down while maintaining reliability, the designs of automotive turbochargers are usually as simple as possible.
Many automotive-size turbochargers incorporate floating bushing journal bearings. These bearings are designed for fully hydrodynamic lubrication at normal operating speeds. For low cost and simple maintenance, turbochargers use the engine oil system for lubrication instead of having a separate system.
The primary consideration in the rotordynamic design of high-speed machinery is to control and minimize vibration. Large-amplitude vibration is undesirable in that it generates noise and can have large amplitudes that cause rotor-stator rub. In most rotating machinery, the dominant vibration is a forced response to rotor imbalance. There exists, however, another class of vibration termed rotordynamic instability or self-excited vibration. Vibration of this type requires a different design approach. Almost all rotors of automotive turbochargers exhibit both forced vibrations and self-excited vibrations (Choudhury, Pranabesh De, “Rotodynamic stability case studies”, International Journal of Rotating Machinery, 2004, pp. 203-211).
Forced vibrations from imbalance are harmonic and occur at the turbo shaft speed. They are generally driven by either mass eccentricity in the rotor or shaft bow. Mass eccentricity is a result of manufacturing tolerances, while shaft bow can be due to manufacturing tolerances or thermal effects. Unbalance vibrations can usually be minimized by designing the rotating element so that no natural frequencies are close to the desired operating speed range. Thermal bowing is the only exception to this previous statement.
Self-excited vibrations usually occur at frequencies that are a fraction, rather than a multiple, of shaft speed. The sub-synchronous vibrations do not require a driving imbalance in the rotating element, but are due to the interaction between the inertia and elasticity of the rotating elements, the aerodynamic forces on the rotor and the hydrodynamic forces in the bearings.
Rotordynamic design of turbochargers has been based on both linear and nonlinear vibration analysis (Holmes, R., Brennan, M. J., and Gottrand, B., “Vibration of an Automotive Turbocharger A Case Study”, Proc. 8th International Conference on Vibrations in Rotating Machinery, Swansea, UK, 2004 pp. 445-450, Li, C. H. And Rohde, S. M., “On the Steady State and Dynamic Performance Characteristics of Floating Ring Bearings”, Trans. ASME Journal of Lubrication Technology, 103, 1981, pp. 389-397, and Shaw, M. C., and Nussdorfer, T. J., “An Analysis of the Full-floating Journal Bearing”, Report No. 866, National Advisory Committee for Aeronautics (NACA)). It was found that floating bushing bearings were more resistant to self-excited vibration than plain journal bearings, and these became widely used. However, with floating bushing bearings many turbochargers show high levels of sub-synchronous vibration (Gunter, E. J., and Chen, W. J., “Dynamic Analysis of a Turbocharger in Floating Bushing Bearings”, Proc. 3rd International Symposium on Stability Control of Rotating Machinery, Cleveland, Ohio, 2005, and Tanaka, M., Hatakenaka, K. And Suzuki, K., “A theoretical Analysis of Floating Bush Journal Bearing with Axial Oil Film Rupture Being Considered”, Trans. ASME Journal of Tribology, 124, 2002, pp. 494-505).
The desire would be to have a small synchronous vibration that would allow the bearing to have less dynamic loading and better performance. It is also desirable to provide a turbocharger capable of higher top speeds with less oil leakage and hence lower emissions.